In that case: vậy thì / thế thì

Vậy thì means "in that case" or "well then". It starts your reply when you draw a conclusion from what someone has just said, usually to suggest or decide something: Em chưa ăn hả? Vậy thì đi ăn với anh. You haven't eaten? Well then, come and eat with me. In Hà Nội it's thế thì.

It's handy at the market. When the seller tells you the price, Vậy thì lấy em hai ký means "In that case, I'll take two kilos" (in Hà Nội, hai cân). In quick speech thì is often dropped: Vậy mai gặp nha. See you tomorrow, then. To spell out the condition, say nếu vậy thì, "if that's the case".

Vậy thì looks forward, to what you'll do now. Vậy là (in Hà Nội, thế là) looks back and tells what happened as a result: Vậy là em trễ chuyến bay. And so I missed the flight.

At the end of a sentence, vậy alone means "then, I suppose", accepting a second choice: Thôi, ăn bún vậy.